As conventional memory cell structures approach scaling limits, other memory cell structures, such as resistive random access memory (RRAM) cells and programmable metallization cells (PMCs) may enable increased miniaturization of electronic devices. Because PMCs provide benefits of low power usage, long useful life, and high memory density, PMCs may replace other forms of memory cell structures in production.
Conventionally, a PMC includes an electrolytic active material and a metal species that reversibly forms a conductive bridge across the electrolytic active material. The conductive bridge may form and dissipate based on an applied electric field, which causes a redox reaction at the electrode interfaces and cationic drift. For this reason, PMCs are referred to herein and in the art as “conductive-bridging RAM” or “CBRAM.” The electrolytic active material includes at least one of a chalcogenide and an oxide. The metal species includes a transition metal, such as silver, copper, or nickel.
Conventional CBRAM devices are fabricated by physical vapor deposition or chemical vapor deposition of the materials (the electrolytic active layer and the metal species) over a substrate. Because the materials are deposited globally, further processing (e.g., chemical etching or chemical-mechanical polishing) is required to isolate individual memory cells. Since many transition metals are not selectively etchable, these processes may remove portions of the metal species from the substrate, making formation of CBRAM with ever-smaller dimensions difficult. For example, silver and copper are difficult to etch with the proper selectivity needed to form CBRAM.
Known methods of forming metal-rich chalcogenides are described in U.S. Pat. No. 6,878,569. The method includes depositing a chalcogenide material, a dopant, and a thin barrier material. The dopant is diffused into the chalcogenide material with UV rays (i.e., photodoping) to form a doped chalcogenide.
U.S. Pat. No. 7,294,527 describes formation of a metal-rich metal chalcogenide, such as a silver-rich silver selenide material, without the direct deposition of the metal or photodoping techniques. Chalcogenide glass is formed over an electrode, and silver chalcogenide is formed over the chalcogenide by physical vapor deposition, evaporative deposition, or sputtering. The deposited silver chalcogenide is treated with a nitrate solution to increase the silver content of the silver chalcogenide.
U.S. Pat. No. 6,890,790 also describes formation of a metal-doped chalcogenide without the direct deposition of the metal or photodoping techniques. Chalcogenide glass and metal are formed by co-sputtering the metal and chalcogenide glass.
To avoid the problems associated with removal of portions of active metal species, methods of selectively forming active metal species for use in CBRAM are desired.